1. Field of the Invention
The present invention relates to a power unit for a motorcycle. In the power unit, a crankshaft is rotatably supported by a crankcase that forms a part of an engine main body to be mounted on a vehicle-body frame. A transmission mechanism is installed in the crankcase. A clutch apparatus to connect and disconnect the transmission of power between the crankshaft and the transmission mechanism. The clutch apparatus is installed in a clutch chamber that is formed between the crankcase and a clutch cover coupled to the crankcase. An oil pump, and an oil filter which is set between an oil pump driven by the power transmitted from the crankshaft and portions to be supplied with oil is attached to the clutch cover. The present invention, in particular, relates to an improved structure for disposing an oil filter.
2. Description of Background Art
Domestic Re-publication of PCT International Publication for Patent Applications No. WO2003-071101 discloses a power unit for a motorcycle in which an oil filter used for the purification of the oil pumped out from the oil pump is attached to a clutch cover. The clutch cover is coupled to a crankcase so that a clutch chamber is formed between the crankcase and the clutch cover. A clutch apparatus is installed in the clutch chamber thus formed for connecting and disconnecting the transmission of power between the crankshaft and a transmission mechanism. The position at which the oil filter is attached to the clutch cover is located above the crankshaft.
In a case where the oil filter is disposed above the crankshaft as in the case disclosed in Domestic Re-publication of PCT International Publication for Patent Applications No. WO2003-071101, there is a problem when increasing the diameter of the cylinder bore for the purpose of obtaining a higher output from the engine. An increase in the cylinder bore diameter brings about the increase in the distance, in the axial direction of the crankshaft, between the axis of the cylinder bore and the oil filter, since the cylinder bore is usually formed above the crankshaft. This results in an increase in the size of the power unit as a whole along the axial direction of the crankshaft. In addition, a timing transmission mechanism, which is placed between a valve lifting apparatus and the crankshaft, is disposed in a position located outside of the cylinder bore at a one end side of the axial direction of the crankshaft. In this case, a structure with timing transmission mechanism being disposed on the same side as the oil filter makes the oil filter to be disposed in a position further outward along the axial direction of the crankshaft, and results in a larger size of the power unit along the axial direction of the crankshaft. To avoid this problem, the timing transmission mechanism has to be disposed on the opposite side to the side where the oil filter is located. Accordingly, for the purpose of preventing the power unit from becoming larger in size along the axial direction of the crankshaft, the oil filter needs to be placed in a position that is as close to the center of the power unit in the axial direction of the crankshaft as possible. Such a placement, however, brings about various constraints on the degree of freedom in design, such as one on the internal diameter of the cylinder bore and another on the location of the timing transmission mechanism.